This invention relates to a method for reading radiation image information stored on a radiation image converting panel, more particularly to a reading method which reproduces correctly said radiation image.
When a stimulable phosphor is irradiated, the radiation energy is stored in said phosphor and the stored energy, when excited with a visible light, etc., emits fluorescence at an intensity corresponding to the stored energy, as is well known in the art.
By utilizing the above characteristic, there has been proposed a method, in which a radiation image information such as human body, etc , is stored as the latent image on a radiation image converting panel having a stimulable phosphor layer (hereinafter abbreviated as converting panel), said converting panel is scanned with an exciting light such as a laser beam, etc., to effect stimulated emission, and said stimulated emission is photoelectrically converted to an image signal, which is then visualized, as disclosed in U.S. Pat. No. 3,859,527 and Japanese Unexamined Patent Publication No. 12144/1980.
In the following, a radiation image information reading apparatus using such a converting panel as mentioned above is described by referring to FIG. 8.
A laser beam 805 for stimulation emitted from a laser beam source 801 is reflected against a galvanometer mirror 804 while being vibrated at a certain amplitude (in the X direction) to irradiate the converting panel 10 having radiation image information stored there on as the latent image.
During this operation, the converting panel 10 is moved at the same time in the Y direction perpendicular to the X direction.
Thus, the converting panel 10 is subjected to main scanning in the amplitude direction (X direction) and to subscanning in the Y direction, whereby the whole region of the converting panel 10 is scanned to effect stimulated emission on the scanning line.
On the other hand, a condensing member 806 (light collecting member) having a light-receiving surface 806a arranged in parallel to the scanning line in the vicinity thereof is provided, said condensing member 806 being gradually narrowed from the light-receiving surface 806a which forms a slender flat section until becoming substantially cylindrical at its terminal end, i.e. the transmitting surface 806b where it is adjacent to a photodetector (e.g. photomultiplier tube, etc.) 807 with a filter 808 for separation of stimulated emission from the laser beam for stimulation sandwiched therebetween.
The stimulated emission which has occured on the scanning line with the arrangement as described above enters through the light receiving surface 806a into the condensing member 806 to reach the transmitting surface 806b, then enters into a photodetector 807 where it is photoelectrically converted and sent to an image display apparatus 811, and the image information converted is processed and observed as a visible image by use of a CRT or a magnetic tape or a light sensitive photographic material.
It should be noted here that, at the light-receiving surface 806a of the condensing member 806, the light from the points within the total reflection angle relative to the light-receiving surface 806a all enters the condensing member 806. Thus, not only the stimulated emission stimulated by the laser beam 805 on the scanning line, but also stray light from outside of the apparatus or a part of the reflected light of the laser beam from the surface of the converting panel 10 or afterglow, etc., are all picked up to be mixed with the stimulated emission displaying the image information to become the noise light which disturbs the correct image information.
Of the above noise lights, the stray light from outside of the apparatus and the reflected light of the laser beam from the surface of the converting panel 10 can be excluded by means of light shielding, filter, etc., and therefore the afterglow from the converting panel 10 becomes the problem as the noise light.
The above mentioned afterglow includes afterglow of fluorescence generated by excitation of the stimulable phosphor of the converting panel with a radiation (hereinafter called fluorescent afterglow) and afterglow of excited emission generated by exciting the energy stored in the stimulable phosphor by a stimulating ray such as laser beam, etc. (hereinafter called stimulation afterglow).
The above mentioned fluorescent afterglow generally exhibits an exponentially functional attenuation curve as shown in FIG. 9. That is, if a radiation is irradiated for .DELTA.t time from the time t1 to t2 and stopped at t2, the emission intensity LO will not be immediately attenuated to 0. The situation of attenuation differs depending on the phosphor, and the time constant before the emission intensity becomes 1/e may be 10.sup.-6 second for a tungstic acid salt, while it may be as long as 10.sup.-3 to 10.sup.-1 second for a phosphor containing rare earth element ions or manganese ions. Also, the fluorescent afterglow will frequently have minor afterglow as represented by the curve b in FIG. 9 overlapped in addition to the major afterglow as represented by the curve a in the same Figure. The aforesaid minor afterglow has generally weak emission intensity, but its time constant for attenuation is markedly great.
While stimulated emission is emitted from a very small area (corresponding to a picture element or pixel) on which a stimulating ray is irradiated at a certain time, fluorescent afterglow is emitted from the whole surface on which a radiation has been irradiated and the light from all the points within the total reflection angle relative to the light-receiving surface 806a of the condensing member 806 in FIG. 8 is all condensed.
In this case, since the condensing area of the condensing member 806 is remarkably greater as compared with the stimulated emission area of the converting panel, even if the fluorescent afterglow intensity per one picture element (one pixel) may become negligibly small as compared with the stimulated emission intensity, the fluorescent afterglow quantity is not negligible as the dose to be transmitted to the photodetector.
For example, if the condensing area of the condensing member is 400 mm .times.2 mm and the size of one picture element is 200 .mu.m .times.200 .mu.m, the number of picture elements condensed in the condensing member is 2.times.10.sup.4, and therefore even if the fluorescent afterglow intensity per one picture element may be about 10.sup.-4 of the stimulated emission intensity, the ratio of the fluorescent afterglow quantity and the stimulated emission quantity of the doses to be transmitted to the photodetector becomes 2 : 1.
Thus, in the radiation image information reading method of the prior art, it is necessary to wait until the fluorescent afterglow intensity becomes sufficiently negligible after irradiation of radiation, whereby it has been difficult to read the image information rapidly and continuously in a large amount. Particularly, this drawback was vital when the time constant of the major afterglow represented by the curve a in FIG. 9 is great or when there exists a minor afterglow with a great time constant represented by the curve b in the same Figure, even though the time constant of the major afterglow may be small.
On the other hand, the above mentioned stimulation afterglow is due to delay in turn-down after stopping of stimulation with a laser beam, and it has an exponentially functional attenuation curve similar to the above fluorescent afterglow as shown in FIG. 10. This attenuation curve differs depending on the stimulable phosphor, but in either case, for example, if excitation is effected from the time t4 to t5 and stopped at t5, attenuation will abruptly occur from the emission intensity SO, but soon after the attenuation rate is dropped to form an exponentially functional curve. The stimulation afterglow has frequently a minor afterglow as represented by the curve d in FIG. 10 overlapped in addition to major afterglow as represented by the curve c in the same Figure similarly as in the case of fluorescent afterglow. The minor afterglow has generally a weak emission intensity, but its attenuation time constant is markedly high.
The stimulation afterglow appears for the first time by irradiation of a stimulating ray, with the progress of scanning by stimulating ray, the stimulation afterglow intensities of the respective picture elements have varied depending on the time from irradiation of the exciting light, and therefore the stimulation afterglow quantity to be transmitted to the photodetector at a certain time is the sum of afterglow quantities in picture elements existing within the condensing area of the condensing member. For this reason, the stimulation afterglow is negligible when the time constant of attenuation of the stimulation afterglow is sufficiently small relative to the scanning time per picture element of the stimulating ray.
Thus, in the radiation image information reading method of the prior art, in the case when the time constant of the major stimulation afterglow represented by the curve c in FIG. 10 is great or in the case when there exists a minor stimulation afterglow with a great time constant represented by the curve d in the same Figure, even though the time constant of the major stimulation afterglow may be small, great stimulation afterglow is added to the stimulated emission which became the noise component of the radiation image, whereby it was difficult to read correctly the radiation image information.
Further, by referring to an example of a converting panel by use of a thallium-activated rubidium bromide phosphor (RbBr Tl) as the stimulable phosphor, this situation is described in detail based on FIGS. 12(a)-(c).
FIG. 12(a) shows a converting panel 10 on which a radiation has been uniformly irradiated. In the Figure, b represents the scanning line of an exciting light (laser beam). H in the same FIG. (b) is a modulation signal of the laser beam and the laser output modulated by said modulation signal and only one line on the converting panel 10 is irradiated with laser. I in the same FIG. (c) is a signal obtained by photoelectric conversion of stimulated emission and stimulation afterglow generated from the converting panel 10 by a photodetector, and J is a signal corresponding to the stimulation afterglow quantity by stimulation of the stimulable phosphor with the stimulating ray. Since the time constant of the stimulation afterglow of RbBr : Tl (time constant : several msec), the stimulation afterglow level is gradually elevated with the progress of stimulating ray scanning and is gradually decreased as shown by the curve K after completion of stimulation. For this reason, as is also apparent from this Figure, the ratio of the true image signal I-J to the signal J corresponding to the stimulation afterglow quantity (SN ratio) is deteriorated with the progress of scanning from b' to b", whereby only about 70 (37 dB) can be obtained at b" of the converting panel 10.
Under the state of the art as described above, methods for improving the drawbacks as described above have been proposed. For example, Japanese Unexamined Patent Publication No. 232337/1984 discloses a method in which an image signal is obtained, which is corrected by detracting the signal corresponding to the afterglow quantity obtained when there is no stimulating ray on the converting panel from the image signal obtained when the stimulating ray is scanning over the converting panel.
However, according to this method, since the term in which the signal corresponding to the afterglow quantity can be detected is limited to the time when the above stimulating ray is not on the converting panel of the reciprocal major scanning period of the stimulating ray, only one detection is possible for scanning of one line at the maximum, whereby no precise correction with respect to the afterglow quantity can be expected.
For, the noise light quantity by the above fluorescent afterglow incident on the light-receiving surface 806a of the condensing member 806 in FIG. 8 as described above will be fluctuated from the fore end to the rear end of subscanning depending on the increase of the surface of the converting panel 10 within the total reflection angle relative to the light-receiving surface 806a and appearance or disappearance of the region with different fluorescent afterglow intensities. Also, the stimulation afterglow incident on the light-receiving surface 806a of the condensing member 806 as described above will be fluctuated as a matter of course with fluctuation in stimulation emission intensity. This is because the noise light quantity mixed during scanning reading will be fluctuated from time to time during the progress of subscanning and main scanning.
This situation is described in detail by referring to FIG. 11. FIG. 11(a) shows recording of, for example, an object to be taken 1101 having a uniform thickness, and FIG. 11(b) shows the relationship between the image signal obtained when the converting panel 10 is scanned with an stimulating ray along the scanning line b and the beam position of the stimulating ray. In FIG. 11(b), the image signal practically obtained by scanning is shown by ml. On the other hand, the signal corresponding to the afterglow quantity is a function of the image pattern recorded on the converting panel 10 and the time (beam position of the stimulating ray) as described above, and changes with time as shown by m3. The corrected (true) imaee signal m2 is obtained by detracting the signal m3 corresponding to the afterglow quantity from the above image signal ml at the respective positions.
However, if here the signal corresponding to the afterglow quantity is detected at the position b1 or b3 of the above scanning line b and this is regarded as the signal corresponding to the afterglow quantity in the region b2 of the above scanning line b (corresponding to m5), the corrected image signal obtained by detracting m5 which has been regarded as the signal corresponding to the afterglow quantity from the image signal ml at the respective positions becomes like m4, which is different from the true image signal m2.
As described above, the fluorescent afterglow quantity and the stimulation afterglow quantity are functions of the image pattern recorded on the converting panel and time, and therefore, it is evident that no precise correction can be made by detecting a signal corresponding to the afterglow quantity only once at the maximum per one line of scanning in spite of their changes from time to time in one line of scanning and correcting the image signal by use of this signal.